The unthinkable suddenly looks possible. Bankers, governments and investors are preparing for Greece to stop using the euro as its currency, a move that could spread turmoil throughout the global financial system. The worst case envisions governments defaulting on their debts, a run on European banks and a worldwide credit crunch reminiscent of the financial crisis in the fall of 2008. A Greek election on Sunday will determine whether it happens. Syriza, a party opposed to the restrictions placed on Greece in exchange for a bailout from European neighbors, could do well.